1. Field of the Invention
The present invention relates to a winding mechanism with tension control function and a trawling apparatus, and specifically relates, to a winding mechanism with tension control function and a trawling apparatus that control a wire to have a constant tension.
2. Description of the Related Art
There is a trawl fishing method in which an operation is performed by towing a trawl net from place to place by a fishing vessel. In the trawl fishing method, a fishing net is connected to wires of winding mechanisms set at both end portions of the rearward of a vessel's stem via otter boards and towed by the vessel. The fishing net can increase captured amounts of fishes and the like by increasing its open area. Accordingly, it is necessary that the tensions of the wires wound off from right and left winches be adjusted.
FIG. 4 shows a schematic structure of a conventional winding mechanism. As shown in the drawing, a fishing net (not shown) is attached to a tip of a wire wound off from a drum 2 of a winding mechanism 1. The winding mechanism 1 includes an oil-pressure motor 3 driving the drum 2 and an oil-pressure pump 4 supplying hydraulic oil to the oil-pressure motor 3, in which a relief valve 5 and a control valve 6 are set in an oil-pressure passage of the oil-pressure motor 3 and the oil-pressure pump 4. In addition, a wire-length sensor 7 and a tension sensor 8 are set in the middle course of the wire being, wound off.
In the winding mechanism 1 structured as above, based on detected signals of the wire-length sensor 7 and the tension sensor 8 of the wire, a control signal controlling the wire to have an appropriate length and tension is sent to the control valve 6 by a control unit (not shown). The oil-pressure pump 4 supplying the hydraulic oil to the oil-pressure motor 3 discharges the hydraulic oil in a manner that the hydraulic oil bypasses at the relief valve 5, so that the hydraulic pressure is adjusted and thereby the tension and the wire length are controlled.
Further, in the trawl fishing method shown in Japanese Patent Application Laid-Open No. 2000-201572, in order to keep the tensions affecting right and left wires substantially equal, current statuses of the tensions affecting the right and left wires are detected based on information detected by various sensors set at the wires. The operating statuses of the winches are then adjusted to thereby keep the tensions affecting right and left winches substantially equal. The adjustment of the operating status of the winch is specifically conducted by keeping the pressure of the oil supplied to the winch substantially constant. In other words, a reference oil pressure is kept by adjusting the pressure of an electromagnetic relief valve to increase or decrease.
[Patent document 1] Japanese Patent Application Laid-Open No. 2000-201572.